The invention relates to columns for the cryogenic separation of gaseous mixtures. More precisely, it relates in particular to columns in which hydrogen can be separated from the CO and from the methane contained in a synthesized gas, by scrubbing the hydrogen with liquid methane to purify it.
It will be noted in what follows that all the pressures mentioned are absolute pressures.
One of the main methods of producing hydrogen on an industrial scale consists in using water vapour to reform a light hydrocarbon such as methane. This yields a hydrogen/CO mixture containing residual methane, which can then be treated in various ways to extract hydrogen in the pure state from it.
In one of these methods, the synthesized gas containing about 70% hydrogen, 25% CO and 5% methane is introduced into a column for the cryogenic separation under high pressure (of the order of 15 to 45 bar) by scrubbing with methane. The synthetic gas is then at a temperature of xe2x88x92180xc2x0 C. The CO and the methane, containing a little dissolved hydrogen, are carried to the bottom of the column in the liquid state and are extracted therefrom, while the hydrogen is extracted from the top of the column in the gaseous state. The CO/methane/dissolved hydrogen mixture is introduced into a second column where it undergoes stripping at medium pressure (of the order of 10 bar). The hydrogen is collected in the gaseous state at the top of the column, and the CO/methane liquid mixture is collected at the bottom of the column. This mixture is then introduced into a column at low pressure (of the order of 2.5 bar). The gaseous CO is collected at the top of the column and the liquid methane is collected at the bottom of the column.
The liquid methane collected from the third column is pumped and introduced in the liquid state into the top of the first column so as to scrub the hydrogen with methane in order to purify this hydrogen. The lower the temperature at which this operation is carried out, the more effective it is. In this particular case, the desire is to keep the temperature in the entirety of the first column as close as possible to xe2x88x92180xc2x0 C., this limit being imposed by the temperature at which methane solidifies, which is xe2x88x92182.5xc2x0 C. For this purpose, as the scrubbing is exothermal, heat has to be extracted from the circulating matter at various points in the column. To do this, it is sensible to use liquid CO produced in a refrigeration cycle.
Certain devices currently in use for this purpose operate on the following principle. The synthetic hydrogen/CO/methane gas is introduced at a given level into the lower part of the column at a temperature of xe2x88x92180xc2x0 C. It passes, as it rises up inside the column, through an active zone of the column, such as a first packing, where it becomes impoverished in CO and heats up to xe2x88x92175.4xc2x0 C. The gaseous mixture is extracted for a first time from the column above this first packing; it is introduced into a first passage of a plate-type heat exchanger where its temperature is brought back down to xe2x88x92180.9xc2x0 C. and is reintroduced into the column above its first point of extraction. It continues to ascend in the column, passing through a second packing where it becomes impoverished in CO and heats up to xe2x88x92176.1xc2x0 C. The gaseous mixture is once again extracted from the column above this second packing and sent to a second passage of the plate-type heat exchanger where it is cooled to xe2x88x92180.9xc2x0 C. then reintroduced into the column above its second point of extraction. As it continues to ascend, it passes through a third packing, after which it is extracted once again from the column to be cooled in a third passage of the plate-type heat exchanger and is reintroduced at xe2x88x92180.9xc2x0 C. into the column above its third point of extraction. As it continues to ascend, the gas passes through a fourth and last packing, above which the scrubbing liquid methane is introduced. As has been stated, the scrubbing of the hydrogen with liquid methane, which causes CO to condense and mix in with the liquid methane causes an increase in the temperature of the ascending gases, entailing extracting them to cool them after each passage through an active zone such as a packing. It is the hydrogen purified of its CO under high pressure which is collected at the top of the column.
Liquid traps with perforated bottoms, placed between the various packings are there to collect the descending liquid leaving a packing and to distribute it over the upper surface of the packing immediately below it. They also make it possible to compensate for the pressure drops in the aforementioned cooling circuits.
The number of packings, the number of levels at which the gases are extracted from the column and to which they are returned after cooling, and the number of passages in the heat exchanger are given by way of indication. The numbers may be higher or lower than those which have been described.
The plate-type exchanger optimally works on the CO collected at the top of the third column, which is in the liquid state at a temperature of xe2x88x92182xc2x0 C. and is vaporized at 2.6 bar under the effect of heat transfers which take place between it and the gases extracted from the first column. The exchanger is supplied with liquid CO by a thermosiphon into which the liquid CO from the third column is introduced and to which the gaseous CO is returned after it has passed through the heat exchanger before being extracted therefrom.
This first column and the heat exchanger and the thermosiphon which are appended to it, constitute a bulky assembly. In particular, the pipework conveying the synthetic gases from the column to the heat exchanger and from the exchanger to the column constitute a complex circuit that it would be desirable to eliminate.
The object of the invention is to propose a column for cryogenic separation, that can be used in a method for producing hydrogen from a mixture containing hydrogen and CO using scrubbing of the gases with liquid methane, which is notably less bulky and less complex than the columns and their auxiliaries customarily used for implementing this method.
To this end, the subject of the invention is a column for the cryogenic separation of gaseous mixtures, characterized in that it comprises:
a cylindrical wall;
means of introducing the gaseous mixture into the said column;
at least one assembly consisting of:
two active zones spaced staggered along the height of the column and filling the entire cross section thereof;
a liquid trap located between the said active zones, consisting of a cylindrical container with a perforated bottom and the outer side wall of which defines, with the cylindrical wall of the column, an annular space in which the gases circulating upwards inside the column can pass after they have passed through the active zone above which the said liquid trap is positioned;
an internal lining of the cylindrical wall of the column, having its lower end located level with the upper edge of a liquid trap, means for causing the gases leaving the active zone located below the said liquid trap to pass into the said internal lining, and means for causing the said gases and the condensed liquid they contain to leave the said internal lining, returning them to the interior of the column above the said liquid trap;
an external jacket of the cylindrical wall of the column, located facing the said internal lining, comprising means of introducing a cooling fluid into its lower part and means of extracting the said fluid at its upper part, the said external jacket allowing heat transfers between the said cooling fluid and the gaseous and liquid matter circulating in the internal lining via the cylindrical wall of the column;
means for introducing a liquefied gas into the upper part of the column to scrub the gas extracted at the top of the column;
means of extracting the most volatile gas from the said gaseous mixture at the top of the said column,
and means of extracting, in the liquid state, the least volatile gas or gases of the said gaseous mixture at the bottom of the said column.
According to a preferred embodiment, the said internal linings comprise, distributed alternately about the internal circumference of the column, first portions which open at their lower part through orifices into the said annular space and through which the gases circulating upwards inside the column can pass, and second portions opening, in their lower part, through orifices above the upper edge of the said liquid trap so as to convey the circulating gases and the liquid which is condensed in the said second portions into the column, the said first and second portions being placed in communication in the upper end of the said internal lining.
The said portions of the internal linings are preferably lined with heat exchange fins of the xe2x80x9cstraight finxe2x80x9d type, in the case of the first portions, and of the xe2x80x9cserrated finxe2x80x9d type in the case of the second portions.
Another subject of the invention is a method for the cryogenic separation of a mixture containing hydrogen and CO, of the type comprising a stage of scrubbing the hydrogen present at the top of the column with a liquefied gas before it is extracted, characterized in that:
it is performed using a column into which the said mixture is introduced;
the cooling fluid introduced into the external jackets of the wall of the column is liquid CO; and
the liquefied scrubbing gas introduced into the upper part of the column is a hydrocarbon.
As will have been appreciated, the invention consists in cooling the ascending gaseous mixture not now outside the column in a separate exchanger, but inside the column itself. This is achieved by means of a series of jackets external to the wall of the column, through which the cooling fluid, such as liquid CO, needed for cooling the gases is circulated. Liquid traps of a suitable configuration and internal linings of the wall of the column direct the ascending gases along the wall of the column, facing the zones at which the external jackets are located, so as to achieve the desired heat exchange. The internal linings also reintroduce cooled matter into the central part of the column.